In the past, electronics communications networks were designed to carry predominantly voice traffic. It is now commonly observed that the growth in computer data traffic far outstrips the growth in voice traffic and that the characteristics of data traffic require a different type of network from that required for voice traffic. Data traffic is characterised by transmission of packets of information, where a packet consists of a header, typically identifying the source and destination of the packet, and payload which contains the data to be transmitted. A number of protocols exist defining the format of the information packet. One well known protocol is known as Internet Protocol (IP) and is increasingly being used for data transmission between end systems. Such networks are generally referred to as connectionless networks.
A typical IP network includes a number of routers or nodes interconnected with communications links. Optical fibres are often used as the physical transmission media for these communications links. Each router examines the headers of the packets which arrive on a given communications link and makes a decision identifying which link to another router or node each packet should next traverse in order to reach its final destination. The forwarding decision is based on the contents of routing tables held locally within each router which indicate preferred routes for particular packet destination addresses. Typically, these tables are generated by a self-discovery mechanism in which each router consults neighbouring routers to establish preferred routes, and this discovery process can take many minutes in large networks.
The routing tables are refreshed periodically. The refresh mechanism allows for reconfiguration as new nodes are added to the network or as failures occur within the network. As a result, an IP network is highly resilient, although restoration of connectivity through re-routing following e.g. a communications link failure can take several minutes to achieve.
In the current ISO standard seven layer model, routers are considered to operate at layer 3, i.e. at the ‘Network’ layer. It is also common practice to build data networks where at least some of the packet forwarding is performed at ISO layer 2 based on layer 2 switches rather than routers.
Failures within a communications network can arise, for example, as a result of equipment failure or as a result of physical severance of the communications link e.g. through earthworks along the route.
Many present IP networks operate on a ‘best efforts’ basis. That is, the communications infrastructure makes no guarantees about whether a message will reach its final destination or, if it does, the length of time it will take to arrive. Whilst this is adequate for many users, some customers, particularly business customers whose profitability depends on timely and assured communications, are demanding service guarantees from their Internet service provider (ISP) and are prepared to pay a higher price for such a service. Economic considerations generally mean that ISPs cannot afford to provide such an assured path universally to all their customers, but only to those who are prepared to pay extra for the service. As a result, many systems now require at least two classes of service: a standard ‘best efforts’ service and a high priority service where information packets take priority over best efforts (lower priority) traffic in the event of congestion in the network and where high priority traffic continues to arrive at its destination even under failure conditions within the network. Whereas using router reconfiguration mechanisms to restore best efforts traffic may be acceptable, the delay incurred in the conventional rediscovery and rerouting mechanism is unacceptable for a high class of service traffic.
A number of mechanisms have been proposed by which the relative priority of different packets can be identified. One such mechanism, defined by the Internet Engineering Task Force is known as ‘DiffServ’. Using this mechanism, a field within the packet header identifies the class of service to which the packet belongs.
In other types of communications networks, for example voice trunk networks, it is common practice to provide duplicate communication paths between node pairs so as to provide standby or protection paths for use in the event of a system fault. A system known as the Synchronous Optical Network (SONET) and a near equivalent known as the Synchronous Digital Hierarchy (SDH) are in common use in optical telecommunications networks. In a typical SONET or SDH system, a number of communicating nodes are connected in a ring wherein each node connects to exactly two neighbouring nodes. In normal use, communication between two nodes uses a defined direction around the ring (for example, along the shorter path). In the event of failure of the connection, e.g. due to equipment failure or fibre breakage along the chosen route, the signal is re-routed in the opposite direction around the ring, thus restoring the connection. Detection of the communications failure and implementation of the re-routing function is performed by the SONET system itself and is normally accomplished within 50 milliseconds. Apart from a brief interruption of service, the connected equipment at the node need not be aware that a failure has occurred and that the traffic is now traversing a different route to the same destination node.
In order to provide such duplicate paths, additional transmission capacity must of course be provisioned in the network. In most SONET systems this additional capacity is reserved for use under failure conditions and is not used to carry useful traffic in the absence of failures. Emerging SONET products allow use of such spare capacity under normal (non failure) conditions, but require more complex schemes to enable this.
In the past, traditional telecommunications companies have accepted the trade-off of spare capacity against the need to provide network resilience. More aggressive financial goals now favour the use of spare capacity to provide an additional revenue stream for the majority of time where the network is operating normally. This is a particular problem for connectionless or Internet protocol (IP) networks where only a portion of the traffic will comprise the significant revenue earning high priority traffic. The total revenues generated by this traffic may be insufficient to support the cost of conventional protection path provisioning to guarantee timely delivery of this traffic.